Good way to refresh databinding on all properties of a ViewModel when Model changes

Question

Short Version

If I update the Model object that my ViewModel wraps, what's a good way to fire property-change notifications for all the model's properties that my ViewModel exposes?

Detailed Version

I'm developing a WPF client following the MVVM pattern, and am attempting to handle incoming updates, from a service, to data being displayed in my Views. When the client receives an update, the update appears in the form of a DTO which I use as a Model.

If this model is an update to an existing model being shown in the View, I want the associated ViewModel to update its databound properties so that the View reflects the changes.

Let me illustrate with an example. Consider my Model:

class FooModel
{
  public int FooModelProperty { get; set; }
}

Wrapped in a ViewModel:

class FooViewModel
{
  private FooModel _model;

  public FooModel Model 
  { 
    get { return _model; }
    set 
    { 
      _model = value; 
      OnPropertyChanged("Model"); 
    }
  }

  public int FooViewModelProperty
  {
    get { return Model.FooModelProperty; }
    set 
    {
      Model.FooModelProperty = value;
      OnPropertyChanged("FooViewModelProperty");
    }    
}

The Problem:

When an updated model arrives, I set the ViewModel's Model property, like so:

instanceOfFooVM.Model = newModel;

This causes OnPropertyChanged("Model") to fire, but not OnPropertyChanged("FooViewModelProperty"), unless I call the latter explicitly from Model's setter. So a View bound to FooViewModelProperty won't update to display that property's new value when I change the Model.

Explicitly calling OnPropertyChanged for every exposed Model property is obviously not a desirable solution, and neither is taking the newModel and iterating through its properties to update the ViewModel's properties one-by-one.

What's a better approach to this problem of updating a whole model and needing to fire change notifications for all its exposed properties?

Answer 1

According to the docs:

The PropertyChanged event can indicate all properties on the object have changed by using either null or String.Empty as the property name in the PropertyChangedEventArgs.

Answer 2

One option is to listen to your own events, and make a helper routine to raise the other notifications as required.

This can be as simple as adding, in your constructor:

public FooViewModel()
{
    this.PropertyChanged += (o,e) =>
      {
          if (e.PropertyName == "Model")
          {
               OnPropertyChanged("FooViewModelProperty");
               // Add other properties "dependent" on Model here...
          }
      };
}

Answer 3

Whenever your Model property is set, subscribe to its own PropertyChanged event. When your handler gets called, fire off your own PropertyChanged event. When the Model is set to something else, remove your handler from the old Model.

Example:

class FooViewModel
{
    private FooModel _model;

    public FooModel Model 
    { 
        get { return _model; }
        set 
        { 
            if (_model != null)
            {
                _model.PropertyChanged -= ModelPropertyChanged;
            }

            if (value != null)
            {
                value.PropertyChanged += ModelPropertyChanged;
            }

            _model = value; 
            OnPropertyChanged("Model"); 
        }
    }

    public int FooViewModelProperty
    {
        get { return Model.FooModelProperty; }
        set 
        {
            Model.FooModelProperty = value;
            OnPropertyChanged("FooViewModelProperty");
        } 
    }

    private void ModelPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        // Here you will need to translate the property names from those
        // present on your Model to those present on your ViewModel.
        // For example:
        OnPropertyChanged(e.PropertyName.Replace("FooModel", "FooViewModel"));
    }
}

Answer 4

    Implements INotifyPropertyChanged
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged 

    RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(String.Empty))

For VB.net if anybody else needs it. If you have already implemented "INotifyPropertyChanged" then the last line is all you need.